Microsoft isn't above a little Google-bashing when it wants to get attention for its own Bing search engine.

The company has launched an ad campaign, dubbed "Scroogled," to call out Google's switch to a pay-to-play system for listings in Google Shopping.

"Sure, they say your search is sorted by relevance, but the truth is, Google sells their shopping results," says a video on Scroogled.com. "They 'scroogle' you by defining relevance as how much they're getting paid."

Naturally, the campaign recommends Bing for "an honest search."

Google Shopping, formerly called Google Product Search, used to list merchants for free. The company announced the switch to paid-only listings in May and started making the transition in October, according to Search Engine Land. Payment is now one of several factors used in ranking search results; when you actually click on a product, all the links to shopping sites on the following page are sponsored.

UPDATE: As the Associated Press points out, Bing gets some of its own listings from Shopping.com, which accepts payments for inclusion. When I asked Microsoft PR about this, I got a statement from Stefan Weitz, Bing's senior director, saying that Bing includes both free and paid listings, but doesn't rank merchants based on who pays. Bing also doesn't let merchants bid on a higher ranking. Google, on the other hand, does factor payments into its shopping search results.

In other words, Bing's approach seems a bit cleaner than Google's, but even it doesn't tell shoppers which results come from paid listings. Neither side is as pure as they'd like you to believe, and as I wrote originally, the end results can be tough to distinguish anyway.

Say what you will about Google's switch, but it's worth pointing out that Bing isn't being completely honest in its claims. Although merchants can influence their rankings by bidding higher on certain search keywords, payment alone doesn't guarantee a high search ranking, as Search Engine Land pointed out last month.

Still, there's no denying that Google has gotten away from its roots of refusing to accept payment for inclusion in search results--and not just for Google Shopping. When users ask Google's main search engine for flight information, they may see a box of suggested flights; some of those listings are paid for by airlines. Although the flight search box is labeled as "sponsored," it appears closer to the main list of blue links, and doesn't have the same yellow shade that typically denotes sponsored results. Indeed, the lines between paid and organic search seem to be blurring at Google, and Microsoft has every right to point that out.

The funny thing about Microsoft's attack ad, though, is that neither search site is a guaranteed great way to find products. The same frustrations apply to both Bing and Google, including duplicate product listings and links to obscure retailers. For all Bing's allegations about "bad search," the two services seem interchangeable in practice.